Turbocharging an engine allows the engine to provide power similar to that of a larger displacement engine. Thus, turbocharging can extend the operating region of an engine. Turbochargers function by compressing intake air in a compressor via a turbine operated by exhaust gas flow. Under certain conditions, the flow rate and pressure ratio across the compressor can fluctuate to levels that may result in noise disturbances, and in more severe cases, performance issues and compressor degradation.
Such compressor surge may be mitigated by one or more compressor bypass valves (CBV) located in the intake. The CBVs may recirculate compressed air from the compressor outlet to the compressor inlet. However, the inventors herein have recognized that recirculation of compressed air may waste the energy utilized to the compress the air, leading to degraded fuel economy under some conditions.
Accordingly, a method is provided to address the loss of energy associated with the recirculation of compressed intake air. In one example, a method for an engine having a compressor comprises generating energy via a turbine-generator positioned in a bypass flow path of the compressor.
In this way, the energy of the recirculated intake air may be recovered by a turbine-generator positioned in the compressor bypass flow path. Flow of air through the turbine-generator may be controlled by a compressor bypass valve. Typically, the compressor bypass valve is controlled to recirculate compressed air in order to avoid surge. During conditions of low flow through the compressor bypass passage, the valve may be controlled to recirculate air through the turbine-generator before reaching the inlet of the compressor. The position of the valve may control the mass flow that is allowed to recirculate around the compressor. This allows a metered amount of air to recirculate, providing compressor operation at a more favorable point on the compressor map. During transient conditions when larger amounts of recirculated air are desired, such as when the intake throttle closes, the valve may be controlled to a position that allows air to bypass the turbine and flow from the high pressure to low pressure side of the compressor via a compressor bypass passage. The compressor bypass passage may be relatively large and/or allow unrestricted flow around the compressor. In doing so, compressor surge may be mitigated under most operating conditions while generating additional energy via the turbine-generator.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.